FIG. 1 illustrates a prior art integrated drive generator air frame power generating system 10 of a type manufactured by the assignee of the present invention for generating three-phase 400 Hz. 120 volt alternating current. The integrated drive generator 10 is driven by a power takeoff 12 from an air frame propulsion engine which varies in speed during operation of the air frame. The power takeoff 12 is coupled to a constant speed drive transmission 14 which functions to produce a constant speed output on shaft 16 while the rotational speed of the power takeoff varies. It should be understood that the construction of the constant speed drive transmission 14 is conventional. Furthermore, the connection of the constant speed drive transmission 14 to the shaft 16 is illustrated only schematically. The integrated drive generator 10 has a permanent magnet generator 18, wound field exciter 20, and main generator 22 each of a conventional construction, having rotors mounted on shaft 16 which is supported by bearings (not illustrated) which are mounted in a housing (not illustrated) of the integrated drive generator. Permanent magnet generator 18 has a permanent magnet rotor 24 mounted on the shaft 16. The stator 26 of the permanent magnet generator 18 outputs alternating current which is rectified by rectifier 28 to produce field excitation current which is applied to the stator 30 of the wound field excitor 20. The rotor 32 of the wound field excitor 20, mounted on shaft 16, outputs alternating current which is rectified by rectifier 34. Rectified current from rectifier 34 is applied to the field windings of the rotor 36 of the main generator 22 to control the output of the main generator. The stator 38 outputs three-phase 400 Hz. 120 volt alternating current for use in powering the various electrical loads on the air frame.
The weight and size of an electrical power generating system is of extreme importance in the design of air frames. Unnecessary weight lessens the overall efficiency of the air frame and its load carrying capability. Increased size in an electrical power generating system can interfere with the mounting of the generator on the propulsion engine when the generator is in the form of an integrated drive generator as a consequence of interference between the integrated drive generator and the cowling of the engine. Shortening of the overall length of the housing of an electrical power generating system with respect to the length of the drive shaft 16 is important in reducing weight, facilitating mounting of the integrated drive generator with respect to the engine cowling and reducing overhung moment which lessens the requirement for reinforcing of the mounting flange on the engine where the integrated drive generator is attached.
Alternative configurations of the electrical power generating system of FIG. 1 exist in which the permanent magnet generator is mounted on a second shaft which results in shortening the overall length of the case containing the generator.
The electrical power generating system of FIG. 1 is operated as a synchronous machine. The main generator 22 has a wound rotor which limits the operating speed of the generator as a consequence of the rotor requiring reinforcement to support the windings on the rotor. Furthermore, the wound field excitor 20 also has a wound rotor having the same attendant disadvantages as the wound rotor in the main generator 22. Finally, the rectifier bridge 34, which rectifies the output current from the wound field excitor 20 and rotates at the velocity of the shaft 16, represents an additional reliability and weight penalty.
Variable reluctance power generators are well known. An example of a variable reluctance power generator is disclosed in U.S. Pat. No. 3,062,979.